Here us one of the mock hemstitching attachments - they all work about the same. you can also use a rug making attachment to get similar results - and some you can get a much wider opening. I have a few of those, but forgot about them . . .
The bottom fabric feeds underneath the attachment
And the top feeds above - you want to make sure you've got some "extra" on the ends to get it started.
Then you sew like normal.
Then fold the seam open and press
I held it up to the window to show the gap better.
And stitch down the edges - however you would like them done. I did one right on the edge, and one about 1/8" away. This picture shows it enlarged.
